Backported from https://github.com/proftpd/proftpd/commit/51b9f9bb6c6dd8ea

--- src/main.c

+++ src/main.c

@@ -481,6 +481,7 @@ int pr_cmd_read(cmd_rec **res) {

   static char *cmd_buf = NULL;

   char *cp;

   size_t cmd_buflen;

+  unsigned int too_large_count = 0;

   if (res == NULL) {

     errno = EINVAL;

@@ -505,8 +506,16 @@ int pr_cmd_read(cmd_rec **res) {

       if (errno == E2BIG) {

         /* The client sent a too-long command which was ignored; give

-         * them another chance?

+         * them a few more chances, with minor delays?

          */

+

+        too_large_count++;

+        pr_timer_usleep(250 * 1000);

+

+        if (too_large_count > 3) {

+          return -1;

+        }

+

         continue;

       }

--- src/netio.c

+++ src/netio.c

@@ -1037,6 +1037,7 @@ int pr_netio_read(pr_netio_stream_t *nst

       }

       nstrm->strm_errno = 0;

+      errno = EOF;

       break;

     }